Depending on characteristics of a lens employed in image pickup, a phenomenon known as shading, in which a peripheral edge portion of an image is darker than a central portion, may occur. JP2005-269373A discloses a technique for reducing noise on the peripheral edge portion of an image by dynamically varying a parameter used by a noise reduction filter in accordance with a correction amount used to correct shading.
JP2010-74244A, meanwhile, discloses a technique for adding shading to an image during image processing without modifying an optical system.
Further, JP2008-293424A discloses a technique of performing noise reduction appropriately in accordance with a frequency of an image by performing multiplex resolution conversion on the image to generate a plurality of band image signals having different frequency bands to each other, and implementing noise removal processing on the respective band image signals.
In a digital image, a discrete luminance is expressed by a numerical value. Therefore, a false contour may be visible on a boundary between a region having a certain luminance and a region having a slightly different luminance. This problem is likely to occur on an image exhibiting shading, or in other words an image on which the luminance varies gradually from region to region.